and Cosmology

3. The World of Galaxies
138
Fig. 3.50. Spectra of galaxies
of different types,
where the spectral flux is
plotted logarithmically in
arbitrary units. The spectra
are ordered according
to the Hubble sequence,
with early types at the bottom
and late-type spectra
at the top
according to the characteristic age of their stellar population
or according to their star-formation rate. Elliptical
and S0 galaxies essentially have no star-formation activity,
which renders their spectral energy distribution
dominated by red stars. Furthermore, in these galaxies
there are no HII regions where emission lines could be
generated. The old stellar population produces a pronounced
4000-Å break, which corresponds to a jump
by a factor of ∼ 2 in the spectra of early-type galaxies.
It should be noted that the spectra of ellipticals and S0
galaxies are quite similar.
By contrast, Sc spirals and irregular galaxies have
a spectrum which is dominated by emission lines, where
the Balmer lines of hydrogen as well as nitrogen and
oxygen lines are most pronounced. The relative strength
of these emission lines are characteristic for HII regions,
implying that most of this line emission is produced in
the ionized regions surrounding young stars. For irregular
galaxies, the spectrum is nearly totally dominated by
the stellar continuum light of hot stars and the emission
lines from HII regions, whereas clear contributions by
cooler stars can be identified in the spectra of Sc spiral
galaxies.
The spectra of Sa and Sb galaxies form a kind of
transition between those of early-type galaxies and Sc
galaxies. Their spectra can be described as a superposition
of an old stellar population generating a red
continuum and a young population with its blue continuum
and its emission lines. This can be seen in
connection with the decreasing contribution of the
bulge to the galaxy luminosity towards later spiral
types.
The properties of the spectral light distribution of
different galaxy types, as briefly discussed here, is described
and interpreted in the framework of population
synthesis. This gives us a detailed understanding of
stellar populations as a function of the galaxy type. Extending
these studies to spectra of high-redshift galaxies
allows us to draw conclusions about the evolutionary
history of their stellar populations.
3.10 Chemical Evolution of Galaxies
During its evolution, the chemical composition of a galaxy
changes. Thus the observed metallicity yields
information about the galaxy’s star-formation history.
We expect the metallicity Z to increase with starformation
rate, integrated over the lifetime of the galaxy.
We will now discuss a simple model of the chemical evo-